In DEP (Direct Electrostatic Printing) the toner or developing material is deposited directly in an imagewise way on a receiving member substrate, the latter not bearing any imagewise latent electrostatic image. The substrate can be an intermediate endless flexible belt (e.g. aluminium, polyimide, etc.). In that case the imagewise deposited toner must be transferred onto another final substrate. Preferentially the toner is deposited directly on the final receiving member substrate, thus offering a possibility to create directly the image on the final receiving member substrate, e.g. plain paper, transparency, etc. This deposition step is followed by a final fusing step.
This makes the method different from classical electrography, in which a latent electrostatic image on a charge retentive surface is developed by a suitable material to make the latent image visible. Further on, either-the powder image is fused directly to said charge retentive surface, which then results in a direct electrographic print, or the powder image is subsequently transferred to the final substrate and then fused to that medium. The latter process results in an indirect electrographic print. The final substrate may be a transparent medium, opaque polymeric film, paper, etc.
DEP is also markedly different from electrophotography in which an additional step and additional member is introduced to create the latent electrostatic image. More specifically, a photoconductor is used and a charging/exposure cycle is necessary.
A DEP device is disclosed by Pressman in U.S. Pat. No. 3,689,935. This document discloses an electrostatic line printer having a multi-layered particle modulator or printhead structure comprising:
a layer of insulating material, called isolation layer; PA1 a shield electrode consisting of a continuous layer of conductive material on one side of the isolation layer; PA1 a plurality of control electrodes formed by a segmented layer of conductive material on the other side of the isolation layer; and PA1 at least one row of apertures. PA1 which comprises different isolated wires as control back electrode PA1 and the toner delivery unit. For a line printer the density can be tuned by selecting an appropriate voltage for shield electrode, control-electrode and control back electrode wire. PA1 a back electrode located-on the receiving member support; PA1 the toner delivery means; and PA1 the common shield electrode of the printhead structure. PA1 the required density value on the receiving member substrate; and PA1 the individual correction parameter per aperture. PA1 a printhead structure 6, at the front side of the receiving member substrate 9, comprising: PA1 wherein each individual electrode 6a or 6b is galvanically isolated from each other electrode; PA1 a toner delivery means 1, at the front side of said printhead structure 6, providing toner particles 4 in the vicinity of said apertures 7;
Each control electrode is formed around one aperture and is isolated from each other control electrode.
Selected potentials are applied to each of the control electrodes while a fixed potential is applied to the shield electrode. An overall applied propulsion field between a toner delivery means and a receiving member support projects charged toner particles through a row of apertures of the printhead structure. The intensity of the particle stream is modulated according to the pattern of potentials applied to the control electrodes. The modulated stream of charged particles impinges upon a receiving member substrate, interposed in the modulated particle stream. The receiving member substrate is transported in a direction orthogonal to the printhead structure, to provide a line-by-line scan printing. The shield electrode may face the toner delivery means and the control electrode may face the receiving member substrate. A DC field is applied between the printhead structure and a single back electrode on the receiving member support. This propulsion field is responsible for the attraction of toner to the receiving member substrate that is placed between the printhead structure and the back electrode.
This kind of printing engine, however, requires a rather high voltage source and expensive electronics for changing the overall density between maximum and minimum density, making the apparatus complex and expensive. Moreover, since not all apertures behave exactly the same, it is very difficult to obtain an image with an overall equal density. This results in a poor output quality, especially in solid areas.
To overcome these problems several modifications have been proposed in the literature.
In U.S. Pat. No. 4,912,489 the conventional positional order of shield electrode and the control electrode--as described by Pressman--has been reversed. This results in lower voltages needed for tuning the printing density. In a preferred embodiment, this patent discloses a new printhead structure in which the toner particles from the toner delivery means first enter the printhead structure via larger apertures, surrounded by so-called screening electrodes, further pass via smaller apertures, surrounded by control electrodes and leave the structure via a shield electrode. The larger aperture diameter is advised in order to overcome problems concerning crosstalk.
In EP-A-0 587 366 an apparatus is described in which the distance between printhead structure and toner delivery means is made very small by using a scratching contact. As a result, the voltage--needed to overcome the applied propulsion field--is very small. The scratching contact, however, strongly demands a very abrasion resistant top layer on the printhead structure.
An apparatus working at very-close distance between the printhead structure and the toner delivery means is also described in U.S. Pat. No. 5,281,982. Here a fixed but very small gap is created in a rigid configuration, making it possible to use a rather low voltage to select wanted packets of toner particles. However, the rigid configuration requires special electrodes in the printhead structure and circuits to provide toner migration via travelling waves.
In U.S. Pat. No. 4,568,955 e.g. a segmented receiving member support comprising different galvanically isolated styli as control back electrodes is used in combination with toner particles that are migrated with travelling electrostatic waves. The main drawback of this apparatus is its limited resolution and dependence of the image quality on environmental conditions and properties of the receiving member substrate.
In U.S. Pat. No. 4,733,256 some of these drawbacks are overcome by the introduction or a printhead structure, as described by Pressman. The printhead structure is located between the receiving member support
In U.S. Pat. No. 5,036,341 a device is described comprising a screen or lattice shaped control back electrode matrix as segmented receiving member support. This apparatus has the advantage that matrix-wide image information can be written to the receiving member substrate, but it also suffers from the environmental influences and those caused by the nature of the receiving member substrate.
To overcome these drawbacks Array Printers described in U.S. Pat. No. 5,121,144 another device wherein the segmented back electrode without printhead structure was changed into a two part electrode system, having a printhead-electrode structure and a back electrode structure. A first part was placed between the toner delivery means and the receiving member substrate and consisted of parallel, isolated wires, being used as printhead structure. A second part consisted of another set of parallel wires, arranged orthogonally with respect to the first wires and was used as back electrode structure. The receiving member support or back electrode structure in all examples consists of isolated wires which are oriented in one direction. As printhead structure, there are described three different configurations:
1. isolated wires in a cross direction; PA0 2. a flexible PCB with only control electrodes in the cross direction; and PA0 3. a flexible PCB with common shield electrode and control electrodes in the cross direction.
The different systems according to this patent make it possible to change the propulsion field in a group of apertures, tuning the density by setting the voltage of the different control electrodes.
According to U.S. Pat. No. 4,491,855 the image density can be enhanced by the introduction of an AC-voltage, applied to the toner conveying member. As a result, shorter writing times are possible. But, to obtain a reduced image density, the same voltage levels must be applied.
In U.S. Pat. No. 5,170,185, a method is described to vary the image density. For that purpose, the voltage, applied to three different stages of the device, can be varied on a time base scale, between a writing time and a non-writing time. These three stages include:
With experimentally obtained variations it is possible to modify the image density obtained by a standard configuration. However, as the different voltages are applied to the back electrode, toner delivery means or common shield electrode, it is not possible to correct a single pixel for a certain density change during a single writing cycle.
In U.S. Pat. No. 5,193,011, a method is disclosed to achieve a pixel by pixel correction by time-modulation of voltages, applied to the different control electrodes around individual apertures. If pixels on the receiving member substrate, imaged by different apertures, exhibit a different visual density, the control electrodes corresponding to these apertures can be driven during different time intervals. As such, a page wide constant image density can be obtained. This method only controls one single electrode per pixel, i.e. the control electrode on the printhead structure. The voltage applied to each control electrode has to take into account:
This means that the correct time modulation must be based on the grey scale value and the density correction.
In U.S. Pat. No. 5,229,794 an apparatus is described which comprises a printhead structure, comprising apertures. Each individual aperture has two distinct electrodes, further on called shield electrode and control electrode. To achieve an enhanced image contrast, two fixed voltages V.sub.1 and V.sub.2 can be applied alternatively to each pair of shield and control electrodes. If V.sub.1 is applied to the shield electrode, then V.sub.2 is applied to the control electrode and vice versa.
All above mentioned patent applications just fulfil one or a few of the different requirements for an inexpensive DEP device, delivering high-quality images with stable densities.
There is thus still a need to have a DEP system, based on a simple apparatus, yielding high quality images in a reproducible and constant way.